Team Comment on "Tiling and Layering Module for SVG 1.2 Tiny (SVGTL)" Submission

Summary

The Submission proposes a set of extensions to SVG, comprised of four main mechanisms:

A standardized coordinate mapping for establishing geographic coordinates within the SVG coordinate space;

A tiling mechanism to allow large large graphical assets decomposed into smaller discrete sub-images to be loaded individually by the client on an as-needed basis, and recomposed client-side to form a composite view of part or all of the original image;

A layering mechanism to allow placement of tiles above or below other tiles along the 'z' axis; and

Properties to define upper and lower bounds of zoom levels for controlling visibility of assets within the viewport.

Collectively, these features provide a Level of Detail solution suitable for, but not restricted to, vector mapping. Some aspects of this proposal will be of general applicability beyond the mapping use case.

Relationships to W3C Activities

The W3C Scalable Graphics Requirements, published in 1996, list the requirements for a Web-oriented scalable graphics solution. Of these, all are satisfied by SVG except for the Level of Detail requirement.

SVG 1.1 (whose Second Edition is now a Proposed Recommendation) and SVG Tiny 1.2 both define how a
single SVG file, representing a map, indicates the projection and map coordinate system used and how this is transformed to graphical coordinates. This Submission extends the idea to allow multiple independent map layers to be georegistered and presented to the user as a composite map.

SVG 1.1 and SVG Tiny 1.2 can reference external vector or raster resources, such as images, but are restricted to downloading an entire resource (although they can clip portions away for display). This Submission extends the idea to allow multiresolution graphics to be declaratively referenced and automatically loaded on demand at the needed resolution in response to user zooming and scrolling. A similar feature, alternate content based on display resolutions , was once proposed for SVG Full 1.2.

Now that SVG is widely implemented, new work is starting on SVG 2. The Tiling and Layering features are candidates for inclusion in SVG 2 to satisfy the Level of Detail requirement.

Implementations

There has been experimental and commercial implementation of this proposal at various stages of its development, including the SVG Map Toolkit, and an open source implementation based on the WebKit browser engine, which will be announced separately.

Next Steps

This submission is timely, as the SVG WG is in the process of rechartering. In the new charter period, we propose to form a task force dedicated to mapping, including defining use cases and requirements for mapping applications, as well as developing Recommendation-track specifications for use in mapping, taking this Submission as input for one such deliverable. The use cases presented by this Submission will serve as input for future work within a dedicated mapping extension to SVG.